Precision variable frequency dividers



INVENTOR.

L. T. THOMASSON Filed April 2, 1956 PRECISION VARIABLE FREQUENCYDIVIDERS Sept. 8, 1959 United States Patent() PRECISION VARIABLEFREQUENCY DIVIDERS Leland T. Thomasson, Seattle, Wash., assignor toBoeing Airplane Company, Seattle, Wash., a corporation of DelawareApplication April 2, 1956, Serial No. 575,552

11 Claims. (Cl. 331-54) vThis invention relates to improvements inelectronic vfrequency dividers andmore particularly in those of the typeincorporating Aa monostable trigger circuit such as a'monostablemultivibrator subjected to recurrent triggering signals of a frequencyto be divided and capable of responding synchronously to selected cyclesthereof constituting a frequency submultiple of said recurrenttriggering signals. The operation of such frequency-division circuitsdepends upon the capacity of the monostable trigger circuit to rejectapplied triggering signals during its quasi-stable period and to respondtothe first such signal applied lthereto immediately followingsubstantial completion of -recovery from its quasi-stable condition inorder to reenter such condition. The invention isherein illustratvelydescribed by reference to the presently 'preferred form thereof Aasapplied to lthe vprecision frequency division of the output of 'a highlyconstant signal frequency source such as a crystal-controlledoscillator; however, it will be recognized that certain changes andmodifications therein may be made Without departing from'the essentialor characterizing features involved.

With conventional frequency division circuits of the type mentionedabove, `it is difficult to achieve stability of operation at frequencydivision ratios of more than ten or twenty to one, with a singlemultivibrator or other monostable trigger circuit as the synchronizedcomponent of the frequency division circuit. Higher division ratios wereachievable in such cases yonly by cascading of monostable triggercircuits having progressively longer quasi-stable periods constitutingsuccessive multiples. In order to obtain stable operation at very highfrequency division ratios a relatively large number of monostabletrigger circuits in cascade arrangement was necessary, involvingnumerous components and the usual problems attending complex andcritically sensitive electronic apparatus.

A principal object of the present invention is practical and reliablefrequency division circuit means capable of stable operation atfrequency division ratios which are many times those achievable withconventional frequency division circuits having the same number ofmonostable trigger circuits incorporated therein. The inventiontherefore provides for relatively simple frequency division circuitmeans having comparatively few components and capable of achieving theoperational stability and accuracy of former circuits encumbered withmany more components.

A related object is to achieve .these purposes in a frequency divisioncircuit which Vis adjustable in its frequency division ratio throughouta relatively wide range.

Another object of the invention is a novel electronic circuitarrangement for accurately synchronizing the recurrent operation of amonostable multivibrator or other trigger circuit at relatively lowfrequency with a definite submultiple frequency of the triggeringsignals produced by ,a relatively high frequency source such as acrystalcontrolled oscillator, whereby recurrent initiation of thequasi-stable condition of the trigger circuit is caused 'to occurconsistently at a given'phase'point in each ofthe submultiple-frequencycycles of 'the triggering signals. The invention further permits'adjusting the divisional frequency of recurrent -operation 'of thevtrigger circuit whil'eretaining'the aforesaid accuracy ofsynchronization thereof 'with the reference source.

`In conventional electronic frequency division circuits 'of thedescribed type the high frequency triggering sig- Vnals are applied'directly 'to `the 'monost'able 'trigger circuit. During thequasi-'stable condition of the trigger circuit the 'applied triggeringsignals have'no'effect; however, Vas the trigger circuit begins torecover 'from the quasi-stable condition 'the exponential decay ofpotential on vthe control `element to which the triggering signals areapplied ultimately nrenders -the circuit Y'responsive to one 'of`thetriggering signals 'so as to retri'gger the Vcircuit into itsquasi-stable condition. The difficulty vwith this conventionalarrangement which causes its instability 'at high Vfrequency divisionLratios is'the fact that the recovery of circuit sensitivity occurs atan exponential rate, 'and the circuit'is retrigg'ered somewhat before'it substantially completes its full recovery from the quasi-stablecondition, namely in the nearly fiat portion of the recovery curve. As aresult, any slight variations in amplitude of the applied triggeringysignals and any slight variations in length of the quasi-stable-period,caused by supply voltage .duc-mations, variations in the characteristicsof Vthe vacuum tubes or other elements used in the trigger circuit,etc., can produce relatively large frequency division errors.

The present invention, recognizing the specific Ynature of the cause ofinstabilities in such circuits provides a 'novel frequency divisioncircuit which includes controlled gate means or the 'like interposedbetween the triggering signal vsource and the vmonostabl'e triggercircuit, which gate means is operated synchronously with the triggercircuit and effectively isolates the .same from the triggering signals`until 'the trigger circuit substantially fully recovers from itsquasi-stable condition on each operating cycle thereof. In the'preferred embodiment of lthe linvention the lquasi-stable triggercircuit comprises a multivibrator, and the novel `gating 'arrangementcomprises an amplifier. The Vincoming triggering signals are applied toone control element 'of the amplifier and Vthe amplifier output islconnected to the monostable trigger circuit for triggering the sameinto its quasi-stable condition. An energy 'storage circuit connected toa second control element in the amplifier is charged during thequasi-stable condition of the monostable trigger circuit to bias off theamplifier. Initiation of recovery of the 'trigger circuit 'from itsquasi-stable condition initiates decay of 'amplifier cut-off bias insuch energy storage circuit at a rate which is materially slower than'the rate of recovery of the trigger circuit, and which progressessubstantially linearly to the point of rendering the ampliiier againoperative. Consequently, a substantially constant period of time isallowed for completion of recovery of the monostable trigger circuitfollowing each initiation 'of recovery thereof from its quasi-stablecondition, before 'the amplifier permits triggering signals to reachsuch trigger circuit, and the former cause of operational instability isthereby eliminated.

The use of the gating amplifier or equivalent means performing thetrigger circuit inhibiting function during the recovery period has thefurther advantage of providing a strong and 4sharply defined triggerpulse to the monostable trigger circuit, which pulse is not affected byadjustment yof the means controlling 'the duration of the quasistableperiod of such trigger circuit which determines the frequency divisionratio of the system.

These Iand other features, objects and advantages of 'ductive and tubeif? is conductive.

' conductor 2?.

the invention will become more fully evident from the following detaileddescription thereof by reference to the preferred embodiment asillustrated in the accompanying drawings.

Figure 1 is a schematic diagram of the frequency division circuitarrangement.

Figure 2 is a wave diagram graphically illustrating the principle ofoperation of the circuit.

Referring to the drawings, the illustrated source of triggering signalscomprises the crystal-controlled oscillator fil delivering a sine waveof constant relatively high frequency to the synchronized trigger pipgenerator 12 from which the recurrent high-frequency triggering signalsemanate. lt will, of course, be recognized that the invention is notconcerned with or confined to any particular circuit apparatus forgenerating the triggering signals the frequency of which is to bedivided. The synchronized trigger pip generator l?. may, for example,comprise a triggered blocking oscillator capable of generating shortdiscrete trigger pulses synchronized cycle for cycle with the sine waveoutput of the crystal-controlled oscillator tu. The output from the pipgenerator l2 is delivered through a damping resistor .1t-'l and couplingcondenser te to the control grid of amplier pentode tube It forming partof the controlled gate amplier circuit.

The control grid of tube i3 is subjected to a negative vbias potentialof a value such that only the sharply de- -ned peaks of thepositive-going triggering signals reach such control grid as a means ofmore precisely phaserelating the amplified triggering signals at theanode of amplifier the original sine waves generated by thecrystal-controlled oscillator iti. Such control grid negative bias isprovided by connecting it to the junction ltetwcen voltage dividerresistors l@ and 26, the opposite end of resistor l being connected tothe conductor 2,2. carrying negative potential minus E3, and theopposite end of resistor 2li being grounded. The amplifier screen gridis connected to a conductor 2d carrying positive supply potential plusEl through the screen loading resistor 25 luy-passed to ground by thecondenser- 23. The amplifier cathode is directly grounded, as shown.

Ampliier tube l and the first tube 30 in the monostable multivibratordraw anode current through a common load resistor 32. The suppressorgrid of amplier l is connected in a voltage divider circuit includingthe resistor 3;?. connected in successive series arrangement with therespective resistors 34 and 36. The junction between the latter tworesistances is connected to the suppressor grid of tube f8, whereas theopposite end of resistance 36 is connected to the negative potentialconductor 22. A storage condenser 33 is connected between ground and thesuppressor grid. The relative values of the resistances 32, 3d and 36are such that with the multivibrator tube 3h substantially nonconductivethe voltage divider potential applied to the suppressor grid of tube i8renders such tube operative as an amplifier of triggering signals. infact, as will be explained subsequently in connection with the wavediagram in Figure 2, such voltage divider potential applied to thesuppressor grid is materially higher than the potential (substantiallyground potential) required to accomplish that result.

The illustrated monostable trigger circuit comprises the multivibratorhaving the two triode amplifiers 3u and lil ,connected in a circuitarrangement whereby in the quiescent condition of the multivibrator,tube 3d is noncon- The cathodes of both amplifiers are grounded. Thecontrol grid of amplifier 3f; is connecte-:l through grid-leakresistance d2 to negative and through coupling condenser 4d, bypassed byresistance 45, to the anode of tube 4l). The latter has a load resistor48 connected to positive conductor 24. rlhe control grid of triode il isconnected through coupling condenser 59 to the anode of triode 39 '-andis connected through lixed resistance 52 in series with l variableresistance 54- to the positive potential conductor 24.

it will be recognized that the combined resistances of resistors 52 andS12- together with the value of coupling condenser 5@ establish theduration of the quasi-stable period of the multivibrator and that byadjusting the value of resistance 54 it is possible to vary suchquasistable period, thereby to vary the frequency division ratio of thesystem. The quasi-stable condition of the monostable multivibrator isinitiated by the negative-going ampli led triggering signals coupledfrom the anode of amplilier il to the control grid of triode through thecoupling condenser Sil. At the inception of such a triggering signaltube itl is rendered less highly conductive, which in turn applies anincreasing potential to the control grid of tube 3i) in trigger circuitfashion. The action is cumulative until tube tu is rendered completelynonconductive and tube 30 is rendered conductive to the point ofsaturation, leaving residual charges on condensers -land Sil. As Lhenegative charge on condenser 5l) leaks olf through resistances 52 and54, a potential is ultimately reached, at the control grid of tube 4l?,at which the latter becomes slightly conductive, and this initiatesrecovery of the circuit from its quasi-stable condition. Recovery is notinstantaneous, however, because it takes time to restore the originalcharges on condensers 44- and Sie, as the anode of triode Sil rapidlyrises in potential with initiation of such recovery action. However,with this rapid rise of anode potential the period of discharge ofenergy from condenser 38 is initiated accompanied by progressive decayof cut-olf potential on the suppressor grid of tube 13. Not untilmultivibrator recovery is substantially fully completed does thedecaying negative potential on the suppressor grid of tube l reach thepoint of cut-off at which amplifier It@ is again rendered operative.

With each initiation of 'the quasi-stable condition of the multivibratora negative impulse is transmitted from the output thereof, namely fromthe anode of tube 3d, through coupling condenser S6 to the anode of tube58 connected in a blocking oscillator circuit. This negative impulsepasses through the primary winding of the blocking oscillatortransformer till, the opposite end of such primary being groundedthrough the by-pass condenser o2. The blocking oscillator is ofconventional forni, including the plate load resistance 64 connecteddirectly to the positive potential point El and through the transformerprimary to the anode of the amplifier triode 5d. r[he transformersecondary is connected at one end to the control grid of amplifier 58and its opposite end is connected to ground through the resistance 66and by-pass condenser d3. The control grid is connected to negative biasconductor 22 through the grid leali resistance '72. The output pulsesfrom the blocking oscillator are derived from the cathode thereof whichis returned to ground through a cathode loading resistance 7) as shown.Thus, with each initiation of the quasi-stable condi 'on of the`multivibrator a positive impulse is developed at the cathode of tube 5Eand this constitutes the final output pulse from the frequency divisioncircuit.

The operation of the frequency division circuit will now be described inconnection with the wave diagrams shown in Figure 2. The latter consistsof five different time diagrams of recurrent voltages occurring in thecircuit depicted in conventional graphical fasi Graph a represents thehigh frequency recurrent triggering nals applied to the control grid oftube itil. Graph b is the voltage waveform appearing at the anode oftube Graph c is the voltage waveform appearing at the suppressor grid oftube f8. Graph d is the voltage form appearing at the control grid oftube Graph e is the output voltage waveform appearing at the cathode oftube 58.

At the beginning of the events depicted in Figure 2 one of thetriggering signals shown in graph a applied to the control grid of gateamplifier tube 18 and amplified and inverted thereby, reaches thecontrol grid :of tube 40, reducing :the conduction in such tube andinitiating vthe quasi-stable condition in the monosta'ble multivibratorcircuit. The anode potential of tube 3i) immediately v,drops to -arelatively low value -as vshown in graph b, and

the potential at the control grid of tube 40 likewise drops to arelatively lo-w `value well below .cut-off value of the tube. The dropin potential at the Vanode of tube 30 produces a charge of energy oncondenser 38 of negative potential with respect tothe suppressor grid oftube 1S, which charge ultimately reaches and thereafter remains at aconstant value during ythe yquasi-stable condition of the multivibrator.As previously explained, a positive-going output pulse appears .at thecathode of blocking oscillator tube 58, as shown in graph e, at theinitiation of the quasi-stable condition in the multivibrator.

As the negative charge on coupling condenser Si) leaks off .through-resistances 52 .and 54 in the exponential fashion shown in graph d apoint is reached, at time t1, at which the monostable multivibratorcommences its abrupt recovery from the quasi-stable condition. Therecovery waveform b', representing primarily the discharge of couplingcondenser Sil follows an exponential curve and is virtually `completed^by the time r2, by which time the slope of .the reooverywave b ispractically zero. ln order to permit the monostable :multivibrator ytocomplete its full :recovery before it can be retriggered by a succeedingtriggering signal, the gate amplifier 1.8 remains Abiased beyondcut-.off throughout the entire period from t1 to t2. At the instant oftime r2, or later, the negative bias potential applied to theVsuppressor grid of gate amplifier 18 has just reached zero in relationto the cathode, or a value at which the amplifier again becomesoperative to amplify the triggering signals applied to its control grid.It is important, however, that the timing of the arrival of lthenegative bias at the suppressor grid of tube i8 be detinitely ,relatedto the point of time t1 at which the multivibrator recovery commences.This is necessary to achieving stability in the frequency divisionoperation of the circuit.

In 4order to cause the amplifier 18 to be rendered operative at adefinite and iixed -time interval after initiation of recovery of themonostable multivibrator, extending beyond the time of .substantiallyfull recovery of such multivibrator, the time constant of the dischargecircuit for the storage condenser 38, and the ultimate potential ec towhich the condenser exponential discharge curve .c' would ultimately .goif the discharge were permitted to complete itself, are so selected thatthe initial linear portion Vof curve -c yintersects the linerepresenting the cut-off bias potential of the supprcssor grid of tube13.

Thus .it will be seen that the chief cause of instability in theoperation of conventional frequency division circuits, namely the.absence of complete recovery of the monostable trigger circuit ormultivibrator, is removed by the action of the gate amplifier indesensitizing the tri-gger circuit during such recover-y period. Inlorder to insure that the interval of desensitization occurs during adefinite time period following initiation of multivibrator recovery, theygate .circuit is controlled by operation of .l the multivibrator itselfand is so devised .that the time delay which it imposes is a preciselyaccurate time period.

I claim as my invention:

1. Frequency division circuit mea-ns comprising, in combination with asource of recurrent timing signals of relatively high frequency to bedivided, mono-stable trigger circuit mea-ns having an output and havinga control input, said trigger circuit means having a stable conditionand an alternate quasi-stable condition of predeterminedv durationmaterially longer than the recurrence period of said timing signals,said trigger circuitv means being triggerable from its stable conditionto its quasi-stable condition by application of a control signal to saidcontrol input following substantially full recov- -eryfof said circuitmeans from its quasi-stable condition, fga'te circuit means :operativelyconnected to said control input to transmit control signals thereto fromsaid-source, and -means connected between said kgate circuit and saidtrigger circuit means and operable to disable said `gate circuit meansfor a period substantially including the period of recovery `of said:trigger circuit means from its quasi-stable condition, thereby toprevent `application of `control signals to said input until after saidtrigger circuit means has substantially fully recovered, said lattermeans including control means energizable by the trigger circuit meansby and during the quasi-stable condition thereof, and delay meansconnected to said control element delaying deenergization of saidcontrol means upon initiation of recovery Yof said trigger circuitymeans from its rquasi-stable condition, thereby to permit substantiallyfull recovery of said trigger circuit means before said gate `circuitmeans permits control signals to again reach said input, said delaymeans imposing a delay interval materially shorter than the :duration ofsaid quasistable condition.

2. The frequency division circuit means defined .in claim 1, wherein thegate circuit means comprises :an amplifier having a control element thecontrol means comprising said control element, and the delay means isconnected to the control velement and comprises an energy storageelement energized by the trigger circuit means during the quasi-stablecondition thereof, and a drainage circuit connected to said storageelement, said delay means vcomprising a resistance in said drainagecircuit prolonging `deenergization of said storage element.

3. Frequency division circuit means comprising, in lcombination, means4comprising a source of recurrent timing signals of a frequency to -bedivided, monostable trigger Icircuit means having an output and having acontrol linput connected to said source to effect triggering of saidtrigger circuit means from its stable condition to a quasistablecondition of predetermined duration `in response `to a signal from saidsource, said trigger circuit means havi-ng a substantially exponentialrate of recovery from its quasi-stable condition, inhibiting meansconnected to said trigger circuit means and operable thereby duringenergization of such inhibiting means to prevent such triggering of saidtrigger circuit means, and energizing means for said inhibiting meansincluding means connecting said trigger circuit means thereto andoperable for maintaining said inhibiting means energized for apredetermined period commencing substantially with initiation 4ofrecovery of said trigger circuit means.

4. The frequency division circuit means defined inclaim 3, wherein theenergizing means comprises an energystorage circuit energized from thetrigger circuit means through the connecting means during thequasi-stable condition of the trigger circuit means.

5. Frequency division circuit means comprising, Vin combination, meanscomprising a source of recurrent timing signals of a frequency to bedivided, monostable trigger circuit means having an output and having acontrol input connected to said source to effect triggering of saidtrigger circuit means from its stable condition to a quasi-stablecondition of predetermined duration in response to a signal `from saidsource, said trigger circuit means having a substantially exponentialrate of recovery during its quasi-stable condition, terminated bycircuit transients preceding full recovery of its stable condition, andgate means interposed in the connections between said source means andsaid trigger circuit means, said gate means having a control inputhaving a normal gate-operating voltage and connected to said triggercircuit means for subjecting the 'latter control input :to Acut- -olvoltage by said tri-ggercircuit means .during said quasistablecondition, thereby to prevent passage of timing signals to said triggercircuit means through said gate means Iduring the quasi-stablecondition, said gate means control input including means producing, uponterminahaving an input connected to the source, having an outputconnected to the trigger circuit means and having a control elementcomprising the control input energizable by the trigger circut means tobias said amplifier for preventing passage of the timing signals, saidcontrol input further including energy storage means having a slowdischarge path delaying deenergization of said control element followinginitiation of recovery of the trigger circuit means.

7. Frequency division circuit means comprising, in combination, amonostable multivibrator circuit having an output and a control inputsubject to energization to trigger said multivibrator circuit from itsstable condition to its quasi-stable condition, and means forrecurringly triggering said multivibrator comprising a source ofrecurrent triggering signals of a frequency constituting a multiple ofthe desired triggering frequency, an amplier having an output connectedto said multivibrator control input, a iirst control element connectedto said source to produce amplified triggering signals in said amplifieroutput, a second control element connected to .said multivibratorcircuit to be energized thereby substantially to bias off said amplifierselectively during the quasi-stable condition of said multivibrator, andenergy storage circuit means connected to said second control elementand including an energy storage element charged during the quasi-stablecondition of said multivibrator, and means forming a discharge path forsaid storage element effecting prolonged discharge thereof commencingwith initiation of recovery of the multivibrator circuit and lastingthroughout substantially the full recovery period thereof to maintainsaid amplifier effectively biased off substantially until the end ofsaid recovery period.

8. The triggering circuit means defined in claim 7, wherein the energystorage circuit means includes a storage condenser connected to theamplifier second control element for biasing oif said amplifier whilestorage condenser charge exceeds a predetermined value, and wherein themeans forming a discharge path for said storage condenser includes aresistance means connected to a point of such relative potential thatthe rate of discharge of said Condenser is substantially linear as theresidual charge thereon reaches said predetermined value.

9. The triggering circuit means defined in claim 7, and means in themultivibrator circuit to adjust the length of the quasi-stable periodthereof without materially changing the rate of discharge of the energystorage means.

l0. Frequency division circuit means comprising, in combination, meanscomprising a source of recurrent timing signals of a frequency to bedivided, monostable trigger circuit means having an output and having acontrol input connected to said source to effect triggering of saidtrigger circuit means from its stable condition to a quasi-stablecondition of predetermined duration in response to a signal from saidsource, said trigger circuit means having a substantially exponentialrate of recovery during its quasi-stable condition, terminated bycircuit transients preceding full recovery of its stable condition, andgate means interposed in the connections between said source means andsaid trigger circuit means, said '8 gate means having a control inputhaving a normal'gateoperating voltage and connected to said triggercircuit means for subjecting the latter control input to cut-off voltageby said trigger circuit means during said quasi-- stable condition,thereby to prevent passage of timing signals to said trigger circuitmeans through said gate means during the quasi-stable condition, saidgate means control input including means producing, upon termination ofsaid exponential recovery of said trigger circuit means, delayedrecovery of the voltage of such control input to the gate-operatingvoltage thereof but materially more rapidly in proportion than the rateof exponential recovery of said trigger circuit means, wherebysubstantially complete recovery of said stable condition is permittedbefore timing signals may again reach the trigger lcircuit controlinput, said gate means comprising an amplifier having an input connectedto the source, having an output connected to the trigger circuit meansand having a control element comprising t-he control input energizableby the trigger circuit means to bias said amplifier for preventingpassage of the timing signals, said control input further includingenergy storage means having a slow discharge path delayingde-energization of said control element following initiation of recoveryof the trigger circuit means, and means operable to vary thequasi-stable condition period of the monostable trigger circuit meanswithout materially changing the predetermined delay period caused bydischarge of the energy storage means.

ll. Frequency division circuit means comprising, in combination, meanscomprising a source of recurrent timing signals of a frequency to bedivided, monstable trigger circuit means having an output and having acontrol input connected to said source to effect triggering of saidtrigger circuit means from its stable condition to a quasi-stablecondition of predetermined duration in response to a signal from saidsource, said trigger circuit means having a substantially exponentialrate of recovery during its quasistable condition, terminated by circuittransients preceding full recovery of its stable condition, and gatemeans interposed in the connections between said source means and saidtrigger circuit means, said gate means having a control input having anormal gate-operating voltage and connected to said trigger circuitmeans for subjecting the latter control input to cutoff voltage by saidtrigger circuit means during said quasistable condition, thereby toprevent passage of timing signals to said trigger circuit means throughsaid gate means during the quasi-stable condition, said gate meanscontrol input including means producing, upon termination of saidexponential recovery of said trigger circuit means, delayed recovery ofthe voltage of such control input to the gate-operating voltage thereofbut materially more rapidly in proportion than the rate of exponentialrecovery of said trigger circuit means, whereby substantially completerecovery of said stable condition is permitted before timing signals mayagain reach the trigger circuit control input, and means operable tovary the quasi-stable condition period of the monostable trigger circuitmeans without materially changing the predetermined period of theinhibiting means during which said inhibiting means remains energizedafter initiation of recovery of the trigger circuit means.

References Cited in the ldie of this patent UNlTED STATES PATENTS2,515,271 Smith et al. July 18, 1950 2,613,318 Snyder et al. Oct. 7,i952 2,660,668 Williams Nov. 24, 1953 2.762.923 Quinn Sept. l1, 1956

